1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and more specifically, to a transistor of a semiconductor device which can operate as a 2-bit or 3-bit cell transistor and a method of manufacturing the same.
2. Discussion of Related Art
Semiconductor memory devices are typically classified into a volatile memory in which stored information is deleted as the supply of power is stopped, and a non-volatile memory in which information is kept although the supply of power is stopped. The non-volatile memory devices may include EPROM (Erasable Programmable Read Only Memory), EEPROM (Electrically Erasable Programmable Read Only Memory), a flash Memory, and the like.
The flash memory device is classified into a NOR type flash memory device and a NAND type flash memory device depending upon the configuration of cells. In the flash memory device, a memory cell that stores data includes cell transistors. Each of the cell transistors includes a control gate and a floating gate. The flash memory device requires some time in storing information since the information is stored using tunneling phenomenon through an insulating film. The NOR type flash memory device is typically used to read a small amount of information in a non-sequential manner at high speed, whereas the NAND type flash memory device is usually used to read information in a sequential manner. However, in a method of storing 1 bit in one cell, the same number of a cell as the level of integration is needed as the degree of integration of a flash memory cell increases. That is, a 64 Mb flash memory device requires 226 cells. Thus, in order to solve this problem, a multi level cell (MLC) has been developed. A method has been developed in which the threshold voltage Vt of the flash memory cell is divided to display states without variations in the flash cell structure. This method has been also developed for a NAND type flash memory device as well as a NOR type flash memory device.
FIG. 1 is a cross-sectional view showing the configuration of a transistor of a typical flash memory device.
Referring to FIG. 1, a conventional stack gate type cell transistor includes a tunnel oxide film 12 formed on a semiconductor substrate 10, a floating gate 14 formed on the tunnel oxide film 12, a dielectric film 22 formed on the floating gate, a control gate 24 formed on the dielectric film 22, a capping film 26 formed on the control gate, and a source/drain region 32 that is formed parallel to the floating gate 14 and over the semiconductor substrate 10 formed below the tunnel oxide film 12. The dielectric film 22 has an ONO (Oxide-Nitride-Oxide) structure in which a first oxide film 16, a nitride film 18 and a second oxide film 20 are sequentially stacked. A thermal oxide film 28 is formed at both sides of the floating gate 14, the dielectric film 22 and the control gate 24. Spacers 30 are formed at the sidewalls of the thermal oxide film 28 and the capping film 26.
In this stack gate type cell transistor, the floating gate 14 is a place where electrons or holes are stored and is insulated by the tunnel oxide film 12 and the dielectric film 22. If electrons are stored in the floating gate 14, the threshold voltage of the stack gate type cell transistor increases. On the contrary, if holes are stored in the floating gate 14, the threshold voltage of the stack gate type cell transistor decrease. Assuming that a state where electrons are stored is defined to be “0” and a state where electrons are not stored is defined to be “1” (or vice versa), data of “0” or “1” that is stored in the floating gate 14 can be normally read in an unlimited manner and the data can be kept completely even when power is out. Therefore, this stack gate type cell transistor can be used as a flash memory cell.
As only one “0” or “1” can be stored in one stack gate type cell transistor, however, this stack gate type cell transistor operates only as single-bit transistor. Accordingly, transistors as many as the number of data to be stored are needed.